The invention relates to a method for the production of biodiesel and biodiesel precursor.
Due to their neutral carbon dioxide balance and improved production processes biodiesel attracts increasing attention as an alternative to conventional petrochemical diesel fuel. In some countries, e.g. within the European Union, diesel fuel must contain a defined amount of biodiesel.
Biodiesel is derived from triglycerides by a transesterification or alcoholysis reaction in which one mole of triglyceride reacts with three moles of alcohol to form one mole of glycerol and three moles of the respective fatty acid alkyl ester. The process is a sequence of three reversible reactions, in which the triglyceride in a step by step reaction is transformed into diglyceride, monoglyceride and glycerol. In each step one mole of alcohol is consumed and one mole of the corresponding fatty acid ester is produced. In most processes performed on industrial scale, methanol is used as the alcohol. However, also biodiesel comprising an ethyl or propyl fatty acid ester is commercially available. In order to shift the equilibrium towards the fatty acid alkyl ester side, the alcohol, in particular methanol, is added in an excess over the stoichiometric amount in most commercial biodiesel production plants. A further advantage of the methanolysis of triglycerides is that during the reaction glycerol and fatty acid methyl ester is produced as the main products, which are hardly miscible and thus form separate phases with an upper ester phase and a lower glycerol phase. By removing glycerol from the reaction mixture a high conversion rate may be achieved.
Details to the manufacturing of Biodiesel may be found at M. Mittelbach, C. Remschmidt, “Biodiesel The comprehensive Handbook”, Graz, 2004; ISBN 3-200-00249-2.
To achieve a defined combustion of the biodiesel it is necessary to decrease the amount of residual mono-, di-, and triglycerides as well as of soaps and glycerol as far as possible. According to DIN EN 14214, biodiesel may contain up to 0.2 wt.-% monoglycerides, up to 0.8 wt.-% diglycerides and up to 0.2 wt.-% triglycerides. In usual practice therefore a water wash is performed to remove soaps as well as residual methanol, glycerol and mono- and diglycerides.
As biodiesel is produced from natural raw materials the concentrations of impurities as well as their composition are varying within wide ranges. This could lead to difficulties during the production of biodiesel. Often smaller amounts of fine precipitations are built which may lead e.g. to filter plugging when biodiesel is cooled down to ambient temperature after production or when stored over a longer period of time. One kind of substances which lead to precipitations in biodiesel produced by transesterification of vegetable oil are glycosides and especially sterylglycosides. Sterines are steroids deriving from cholesterine which have a hydroxyl group only at the position C-3, but apart from that do not have any functional group. Mostly they show a double binding in the 5/6 position, more seldom also in positions 7/8 or 8/9. Formally they represent alcohols and therefore are often designed as sterols. Naturally occurring sterylglycosides often comprise—beside the glycoside bound sterine—a fatty acid with which the primary hydroxyl group of the sugar is acylated. Therefore they are very well soluble in vegetable and animal oils and fats. These non-acylated sterylglycosides are nearly insoluble in biodiesel. Therefore they are present as very fine floating particles which can for instance act as germs for the crystallisation of other compounds. Consequently, difficulties caused for example by monoglycerides still present in the biodiesel can be reinforced. Non-acylated sterylglycosides in very small concentrations can already lead to precipitation of solid aggregates from the biodiesel. At room temperature concentrations in a two-digit ppm range can already cause the formation of turbidities in the biodiesel. Non-acylated sterylglycosides show a very high melting point of around 240° C. Therefore, turbidities or precipitations caused by non-acylated sterylglycosides cannot be simply solved by heating up the biodiesel to a higher temperature. When there are already deposits on the filter, the filter will be completely plugged within a short time when non-acylated sterylglycosides are present in the biodiesel.
After the production process the biodiesel is finally investigated. When it is stated that the test with respect to the filter plugging is not passed, because the actually final biodiesel still contains small amounts of non-acylated sterylglycoside, this biodiesel cannot be released.
A process, known from the state of the art, for the separation of components as for example sterylglycosides from biodiesel, is based on cooling down the crude biodiesel to low temperatures and subsequent filtration. Alternatively, the precipitation deriving from sterylglycosides can be removed by centrifugation. These processes, however, are very time and work-intensive.
Another critical contamination of the biodiesel is phosphorous in the organically bound form. It is introduced into the biodiesel by phosphatides (phospholipids). The current European legislation (EN 14 214) limits the P content of the final biodiesel to 4 ppm. In addition, it is to be expected that the limit for phosphor and also sterylglycosides will be further decreased.
In the current biodiesel production the phosphatides are removed at the stage of the oil. This means that the producers of biodiesel need either to employ/purchase oil with a low phosphatide content or to pre-treat the oils. The removal of the phosphatides from the edible oil is called degumming. This process as currently practised within the state of the art involves the treatment of the crude oil with water and/or an aqueous acid. This treatment will, however, not sufficiently remove the phosphor-compounds to obtain an oil which is—after a biodiesel production process—already compliant with the current EU norm EN 14214.
Adsorbents for the phosphatide removal and the reduction of metal ions from vegetable oils are already available on the market, as for example a silica gel based adsorbent of the company Grace which is sold under the brand name Trisyl®. Furthermore bleaching earth based adsorbents are available on the market from Süd-Chemie AG, Moosburg.
The patent EP 0185182 B1 (equivalent to U.S. Pat. No. 4,629,588) from Grace describes an amorphous silica gel with pore diameters between 60 and 5000 Å being suitable for the removal of metal traces from triglycerides. Here it is especially pointed to the application in soybean oil.
In a series of further patents Trisyl® is now combined with other adsorbents resp. the amorphous silicium is combined with other patents resp. further surface modified. So, the patent EP 340717 A2 from Grace describes a two-phase (two-step) adsorption resp. treatment of vegetable oil, whereby the oil is contacted with an amorphous silica gel in the first step in order to remove the phospholipids and soaps. In a second step it is conducted through a packed bed of bleaching earth in order to remove the pigments (coloured bodies). Other patents covering the use of silica in degumming are EP 295418 B1 from Grace, EP 507217 A1 from Grace and EP 0507424 A1 from Grace.
The consequences of the sterylglycoside content on the quality of biodiesel and the analytics of sterylglycosides are described e.g. in: Identification and Occurrence of Steryl Glycosides in Palm and Soy Biodiesel, V. van Hoed et al., J. A, Oil Chem. Soc. (2008) 85, 701-709, in: The Identification and Quantification of Steryl Glycosides in Precipitates from Commercial Biodiesel, R. A. Moreau, K. M. Scott, M. J. Haas, J. Am. Oil. Chem. Soc. (2008) 85, 761-770 and in Fuel properties and precipitate formation at low temperature in soy-, cottonseed-, and poultry fat-based biodiesel blends, Tang, H. et al., Fuel 87 (2008) 3006-3017. Additional details can be found in the Patent WO 2010102952 (A1) from Novozymes.
Sterylglycosides are glycosides from a sterol, e.g. sitosterol and a sugar, e.g. glucose. In plant tissues and in vegetable oils, sterylglycosides occur naturally in both free sterylglycosides (FSG) and acylated sterylglycosides (ASG) forms. In the latter, the 6-position of the sugar is esterified with a long chain fatty acid. Under alkaline conditions, this ester bond between the glucose and the fatty acid is broken, and an acylated sterylglycoside is converted into its free form. Such a side reaction occurs during transesterification, resulting in an increased FSG concentration in biodiesel in comparison to their initial amount in the feedstock oil.
Modern engines feature a sophisticated design involving fine openings for fuel injection, protected by dedicated filters. The content of insoluble contaminants in biodiesel is a closely monitored parameter, since an excess of them might cause operational problems in vehicles due to clogging of the engine filters.
Excessive sedimentation may occur in biodiesel well above its cloud point. This phenomenon is frequently detected in soy and palm biodiesel and induces a number of undesired consequences at both the production and quality control stages. In the beginning of crystallization a cloud of tiny particles is dispersed through the entire volume of the biodiesel. It causes a hazy appearance of the product, marked by the loss of transparency and brilliancy. As sedimentation progresses, deposits are formed on the bottom of biodiesel storage tanks. In particular cases the haze manifests itself within a short time delay after biodisel production and at rather high temperatures (60° C.). Then the progress equipment upstream of the tank farm is affected, and frequent maintenance of fouling heat exchangers and centrifuges may be necessary. As a result, the haze impedes the product from meeting the requirements on contamination/filterability according to the biodiesel quality standards, e.g. European norm EN14214 and ASTM D6751 adopted in the US.
Recently, the problem of deposits on plugged vehicles filters was linked to the presence of free sterylglycosides (FSG) in blended fuel systems.
Accordingly, the removal of sterylglycosides from biodiesel is therefore often necessary and a few methods based on filtering or adsorption are indicated in WO 2007/076163 which describes a process for treating biodiesel by contacting it with a compound being capable of removing sterylglycosides from the biodiesel by adsorption; US 2007/0175091 which describes a method for removing impurities from biodiesel comprising: (a) converting a feedstock into biodiesel having a temperature exceeding 98° C.; (b) cooling the biodiesel to a temperature range sufficient to form particulates of impurities; and (c) filtering the cooled biodiesel to remove the particulates; and WO 2008/051984 which describes a method of passing a biodiesel stream through a filter having a molecular weight cut-off of less than 1,000,000 g/mol.
However, these methods of physically removing sterylglycosides are associated with a yield loss of biodiesel. Therefore there is still a need for alternative processes to remove sterylglycosides from biodiesel with low yield-loss, to provide products that are able to meet the biodiesel quality standards on contamination and filterability and which do not suffer from fuel filter plugging problems.
One alternative are enzymatic processes for biodiesel treatment as described e.g. in WO 2009106360 A2, the other is to remove the sterylglycosides already from the vegetable oil together with the degumming.
The publication “Glycolipids of the Recovered Palm Oil from Spent Earth in the Physical Refining Process” by M. Yamaoka, P. Jenvanitpanjakul, A. Tanaka, J. Jpn. Oil Chem. Soc. Vol. 38, No. 7 (1989), pp 572 describes the enrichment of glycolipids in spent bleaching earth.
For the efficiency of the entire production process of biodiesel from edible oil, it is a drawback to remove phosphatides in oil in a first step and—at a later stage and in a second step after the production of biodiesel—to lower the content of sterylglycosides in the biodiesel. Therefore the entire process would be much more efficient if at the stage of the oil the sterylglycosides could be removed together with the phosphatides in one step. Further, in case there would be a possibility to remove non-acylated but also acylated sterylglycosides from the crude oil, no formation of new non-acylated sterylglycosides would take place during the biodiesel production process.